Fix mtime for tar command and use numeric user/group
Epoch values have to be preceded by '@'. Otherwise it seems that tar
sets a time stamp of -2^63, which we definitely don't want.
Since we use --numeric-owner, we might as well use 0 instead of root.
Follows the advice of https://reproducible-builds.org/docs/archives/,
this time correctly.
Reviewed By: kfunk
Differential Revision: https://phabricator.kde.org/D13832